1. Field of the Invention
The present invention relates to power saving techniques for high speed transmitters.
2. Related Art
In conventional gigabit (e.g. Ethernet) transmitters, current-mode digital to analog converter (DAC) architecture is implemented together with power saving features. These power saving features allow the transmitter to efficiently operate in different modes, namely class-A, class-AB, and class-B modes. In a current-mode transmitter, the differential output current defines the transmitted symbols. Desirably, common-mode current does not contribute to the definition of the transmitted symbols. Also, in these conventional transmitters, the common-mode current is desirably kept as low as possible to minimize the power consumption. Ideally, the common-mode current should be zero.
In a conventional analog front end (AFE), which typically includes a conventional transmitter, inputs of the receiver are connected through a hybrid (echo canceller) circuit to the transmitter's outputs. This connection typically occurs across a duplex communications channel, such as those used in gigabit units. The hybrid circuit cancels out the echo signals traveling back through the output of the transmitter to the receiver inputs. This hybrid circuit, however, is only effective in canceling the differential signal. It does not cancel any undesirable common-mode signals.
The transmitter DAC (TXDAC) that operates in a class-AB or a class-B mode substantially varies its output common-mode voltage as its idle output cells are operated in lower standby (common-mode) current to save power. In class-A mode, the TXDAC is operated at a constant common-mode current that contributes no common-mode voltage variation, but consumes more power. In conventional TXDACs, none of the currently available class-AB, class-B, nor class-A modes, are considered to be efficient from a power savings perspective.
What is needed, therefore, is a method and system that provides an efficient common-mode voltage suppression scheme that will facilitate more efficient class-AB, class-B, and class-A operation in TXDACs. It is desirable that such common-mode voltage suppression techniques, implemented within the TXDAC, will alleviate the need for the AFE's receiver to reject common-mode voltages.